In recent medical field, electronic endoscopes are frequently used for diagnoses and treatment. The electronic endoscope has a probing portion that is inserted into a body cavity, such as stomach, of a subject under inspection, and an imaging unit including a CCD or the like is incorporated in a distal end of the probing portion. The electronic endoscope is also connected to a light source unit, so that light from the light source unit is projected from the distal end of the probing portion to illuminate the inside of the body cavity. While the inside of the body cavity is illuminated, subject tissues inside the body cavity are imaged by the imaging unit. Captured images are processed in various ways in a processor, which is also connected to the electronic endoscope, and the processed images are displayed on a monitor. The electronic endoscope thus allows viewing images of the inside of the body cavity of the subject under inspection in real time fashion, enabling the doctor to make exact diagnoses.
The light source unit generally uses a white light source, such as a xenon lamp that emits white light having a broadband wavelength range from the blue ray region to the red ray region. Using the white broadband light for illuminating the body cavity allows capturing such an image that is useful for observing the whole subject tissues inside the cavity. However, the image captured under the broadband light is indeed effective for rough perception of the subject tissues, but insufficient for observing the details of capillaries or microscopic vessels, deep blood vessels, bit-patterns (gland orifice structure), and surface asperity of the subject tissues, such as concaves and convexes. It is known in the art that the details of the subject tissues will be more visible when illuminated with narrowband light having a limited wavelength range. It is also known in the art that various kinds of information about the subject tissues, such as arterial and venous oxygen saturation levels, may be acquired from image data obtained under the narrowband illumination light, and the acquired information may be graphically displayed.
For example, Japanese Patent No. 3559755 discloses projecting sequentially three kinds of narrowband rays: the red ray, the green ray and the blue ray, to capture an image during each projection period of the ray of one kind. Because the ray of longer wavelength can reach deeper inside the tissues, and the wavelengths of the blue, green and red rays get longer in this order, an image of superficial blood vessels may be obtained during the blue ray illumination, an image of middle-layer vessels may be obtained during the green ray illumination, and an image containing enhanced deep blood vessels may be obtained during the red ray illumination. This prior art also discloses processing the respective images obtained during the separated color illumination, to produce an image showing the superficial blood vessels, the middle-layer vessels, and the deep blood vessels in different colors from each other.
Japanese Patent No. 2648494 discloses projecting three kinds of narrowband infrared rays IR1, IR2 and IR3, wherein the rays IR1 and IR3 are of such infrared regions that the light absorbance of blood vessels to the rays IR1 and IR3 will change according to the change in oxygen saturation of blood, whereas the ray IR2 is of such an infrared region that the light absorbance of blood vessels to the ray IR2 will not change regardless of oxygen saturation of blood. An image is captured during each projection period of the ray of one kind. On the basis of images captured under the illumination of the narrowband rays IR1 and IR3, to which the light absorbance of the blood vessels changes with the oxygen saturation, and an image captured under the illumination of the narrowband light IR2, to which the light absorbance will not change, variations in luminance between these images are calculated. The calculated luminance variations are reflected in an image to show the variations as gray-gradations or artificial color variations, so the image provides information about the oxygen saturation in the blood vessels.
In Japanese Patent No. 2761238, an endoscope captures one image while projecting a narrowband ray of a wavelength range around 650 nm, to which the light absorbance of the vessels will change according to the change in oxygen saturation, and other images while projecting a narrowband ray of a wavelength range around 569 nm light and a narrowband ray of a wavelength range around 800 nm, to which the light absorbance of the vessels will not change regardless of the oxygen saturation. Base on these images, information on the distribution of the hemoglobin amount and information on the oxygen saturation are simultaneously acquired, to produce a color image reflecting these two kinds of information.
There has recently been a demand for such a technology that makes the depth and oxygen saturation of the blood vessels perceivable at the same time on making diagnoses, treatments or the like. However, acquiring information about the blood vessel depth and the oxygen saturation at the same time has been difficult because of many factors, for example, because the light absorbance of hemoglobin in the blood vessels obviously changes depending on the wavelength (see FIG. 3), although simultaneous detection of the hemoglobin amount and the oxygen saturation can be achieved using illumination rays of different narrowband ranges, as disclosed in the above-mentioned Japanese Patent No. 2761238.
Projecting the three narrowband rays of red, green and blue, like in Japanese Patent No. 3559755, may provide information about the blood vessel depth, but cannot provide information about the oxygen saturation. On the other hand, projecting the narrowband infrared rays IR1, IR2 and IR3, like in Japanese Patent No. 2648494, may provide information about the oxygen saturation, but cannot provide information about the blood vessel depth. Even with those rays of wavelength regions which meet both conditions defined in the Japanese Patents Nos. 3559755 and 2648494, it is hard to acquire information about the blood vessel depth and information about the oxygen saturation at once.
The present invention is provided in view of the foregoing problem, and has an object to provide an electronic endoscope system and a processor for an endoscope, which allow acquiring information about the blood vessel depth and information about the oxygen saturation as well. The present invention also has an object to provide a method of displaying these two kinds of vascular information at the same time.